1. Field of the Invention
This invention relates to the process and apparatus for making silicon imide which is useful as a precursor for making silicon nitride and, more particularly, to a process which achieves economical control of the exothermic reaction between silicon halide and ammonia without contamination of the resulting precursor by organics.
2. Description of the Prior Art
The prior art processes have utilized various modes of synthesizing silicon nitride precursor for making silicon nitride by thermal decomposition. The various prior art modes for making such precursor have utilized the reaction of SiCl.sub.4 with ammonia either at low or high temperatures. Such modes can be classified generally as follows: (a) a reaction between a liquid phase containing SiCl.sub.4 and liquid ammonia either at normal ambient pressure conditions or at elevated pressure conditions; (b) vapor SiCl.sub.4 and gaseous ammonia at highly elevated temperatures; (c) a liquid phase containing SiCl.sub.4 and gaseous ammonia; and (d) condensed vapor SiCl.sub.4 and solid ammonia.
The first mode is best represented by the teachings of Ube Industries, Ltd. (disclosed in U.S. Pat. No. 4,196,178) wherein a metallic halide is reacted with liquid ammonia in the presence of an organic solvent. Unfortunately, the presence of the organic solvent contaminates the imide product, the organic solvent being so necessary for this patent to control the extremely exothermic all liquid reaction when operating about ambient or lower temperatures; the evolution of a considerable amount of heat makes the reaction highly uncontrollable and unsuitable for scale-up when organics are not present. Without some agent for control, the low temperature reaction between liquid SiCl.sub.4 and liquid NH.sub.3 is violent. The highly exothermic reaction is seen as responsible for several processing difficulties such as loss of temperature control, nonuniform product, and inlet port clogging. Additionally, scale-up of the reaction of liquid SiCl.sub.4 with liquid ammonia suffers due to limitations of heat extraction for larger equipment from greater heat evolution from greater volumes.
Other authors have also turned to diluting SiCl.sub.4 in an organic liquid (such as benzene, hexane or toluene) before contacting the liquid ammonia. Such dilution of SiCl.sub.4 with an organic which moderates the reaction, and has been described by: (1) Ebsworth, "Volatile Silicon Compounds", Pergamon Press, Ltd., MacMillan, New York 1963, p. 116; and (2) Sato in Japanese Kokai Tokkyo Koho No. 79,134,098, Apr. 11, 1978, listed in Chem. Abstracts as 92:113132n, "Silicon Nitride". In the latter reference, SiCl.sub.4 is diluted with CCl.sub.4 before reaction with ammonia. The dilution allows time for the heat of reaction to be carried away from the reaction interface. Unfortunately, it also brings the organic liquid into contact with the newly formed high surface area imide, which contact results in carbon contamination of the silicon nitride product and which contamination affects, in technically undesirable ways, the dielectric, optical and high temperature phase equilibria within silicon nitride and its alloy compositions. Carbon contamination is also detrimental to second phase oxynitride development and thermal stability.
To avoid the presence of the organic solvent, U.S. application Ser. No. 812,036, assigned to the assignee of this invention, teaches that using an excess of liquid ammonia while conducting the reaction at critically low temperatures, while continuously removing a certain portion of the liquid ammonia containing dissolved byproducts of the reaction, will tend to moderate the thermal reaction but still require considerable cooling at subcritical temperatures; the latter can be highly troublesome from a commercial standpoint.
At high reaction temperatures, necessary for mode (b), undesired bonding of the halide (such as chlorine) to silicon and hydrogen ions will occur (see Mechalchick in U.S. Pat. No. 4,145,224; Kato et al "Finely Divided Silicon Nitride By Vapor Phase Reaction Between Silicon Tetrachloride and Ammonia", Yogyo Kyokaishi 80 (3) 28-34 (1972). Such a high temperature reaction produces hydrogen and chlorine contaminated silicon nitride rather than the imide precursor. If the contaminated nitride is heated sufficiently (around 1500.degree. C.) to break the N-H and Si-Cl bonds, an agglomerated fibrous powder will result, requiring extensive milling before ceramic processing use, the milling adding consequential milling media contamination.
With respect to mode (c), liquid silicon tetrachloride has been reacted with an excess of ammonia gas in dry, deoxygenated benzene or normal hexane at about 0.degree. C. as described by Mazdiyasni in U.S. Pat. No. 3,959,446. This process suffers from the same difficulties described in connection the use of organic solvents in a liquid to liquid reaction. In the French article by E. Vigouroux and C. R. Hugot, Seances Academy Sciences, Vol. 186, p. 1670 (1903), liquid silicon chloride was reacted with gaseous ammonia at a low temperature. This mode generates enormous heat, but the authors never state the particular temperature range of the reaction during processing. The review of the technical literature by these authors indicates that there is a considerable lack of control of the reaction and the various byproducts resulting therefrom. Their research work corroborates such lack of control.
The last mode (d), is disclosed in the article by O. Glemser and P. Naumann, "Uber den Thermischen Abbau Von Siliciundiimid Si(NH).sub.2 ", Z. Anorg. Allg. Chem., 298 134-41 (1959). In this German article, SiCl.sub.4 is transported and condensed as a solid on solid ammonia at minus 196.degree. C. Tremendous heat is generated as the ammonia begins to thaw. The reaction is highly uncontrollable because upon the thawing of the ammonia, the only way to control the reaction is to limit quantities of reactants. The process is converted into a series of complex reversion steps, whereby the reaction flask is warmed to near boiling point then cooled again to maintain some degree of temperature control. Complexity of heating and cooling, stopping and starting, is only possible on a laboratory scale and cannot be reasonably used in commercial production.
The ability to economically control the thermal reaction must be solved with a view towards maintaining high purity of the resulting intermediate imide product of such reaction. Heretofore, attempts to solve the thermal control problem have resulted in greater contamination such as carbon contamination through use of organic liquids (such as benzene, hexane or toluene). Similarly, when the reaction was elevated in temperature to carry out vapor reactions, chloride and hydrogen contamination, with strong bonding between silicon and chlorine, and between nitrogen and hydrogen, occurred with such high temperatures reactions. Thermal decomposition of the product at extremely high temperatures broke the bonding between the nitrogen, hydrogen, silicon and chlorine bonds, but resulted in agglomerated, fibrous powders requiring extensive milling before ceramic processing use.